Volume 12 issue 2
What’s Happening at Missouri S&T (formerly UMR):
Short Course Dates
We will be offering "Basic Composition of Coatings" March 16-20 (Spring 2015). The Basic Composition course is intended for new personnel in the coatings profession. It targets the components of coatings (resin, pigments, extenders, solvents and additives), testing and specifications, general formulation and manufacturing methods. Basic Composition is primarily a lecture course with several laboratory demonstrations.
We will be offering "Introduction to Paint Formulation" May 18-22 (Spring 2015). This course is intended to give the person a fundamental knowledge of how to approach a starting formulation and troubleshoot it. This course involves both lecture and laboratory work.
Online Short Course
We are offering "Introduction to the Coating Systems" online short course. This course is targeted for automotive and aviation type OEM companies. This self-paced seminar will cover the painting system from the composition of paints to the evaluation of the dry film. The pigments, resin, solvents and additives will be discussed including their influence on the coatings performance. Color measurement, surface profile, and other evaluation criteria will be related to composition. The importance of surface preparation and other manufacturing criteria will show the system complexity and each step's importance.
We are offering "Surface Defects: Elimination from Human and Process Contaminants" online short course. This course addresses many of the issues in prevention and minimization of defects. The course covers the defects caused by the coatings process, as well as human issues, including personal care product causes. Several of the surface defects are discussed – from basic principles and real world automotive and aircraft examples. The highly practical approach of this course will greatly aid the personnel involved in the painting operation to reduce and systematically approach issues.
We have started an employment section for our students and companies. We have a full time job section, an intern / co-op section and a graduating and alumni students section . Please explore our section on employment on our web site. Anyone wanting to have job opening listed, please contact us at (573) 341-4419 or e-mail: email@example.com . You can also write to us at Missouri S&T Coatings Institute, BOM #2, 651 W. 13th St., Rolla, MO 65409-1020. Our web site is http://coatings.mst.edu
Technical Insights on coatings Science
Selective property enhancement by modification of epoxy
Sagar Vijay Gade (S&T-Student), Missouri University of Science and Technology
The epoxy resin was commercialized in the late 1940s using bisphenol-A (BPA) which is still the work horse of the epoxy industry. Epoxies are easy to cure over a wide range of temperatures depending on crosslinker. Epoxy coatings do not significantly shrink as there is no byproduct or water formation during curing. They exhibit excellent adhesion properties imparted from their polar hydroxyl, ether and amine groups while the aromatic part gives better thermal stability and chemical resistance. Basic epoxy resins (DGEBA) can be modified to alter or improve the properties of the final coating. Many improvements have been made by chemical modifications of the traditional epoxy resin and utilization of new crosslinking agents.
Commercially epoxies are synthesized by reacting bisphenol A (BPA) or bisphenol F (BPF) with epichlorohydrin (ECH) in excess to produce epoxide terminated resin. The ratio of BPA and ECH controls the molecular weight and hence epoxy equivalent weight. The epoxy resins are commonly crosslinked with amines but phenols, carboxylic acid, anhydrides, hydroxyl and mercaptans can also be utilized.
The properties of epoxy coating depend on crosslinking density. The low molecular weight epoxy has lower epoxy equivalent weight i.e. higher number of epoxide functional groups available for crosslinking compare to high molecular weight epoxy. By using low molecular weight epoxy resin, higher crosslinking is achieved which imparts higher toughness and hardness to the coating, but these coatings are brittle in nature. If high molecular weight epoxy resin is utilized, the final crosslinked epoxy coating has better flexibility and impact resistance due to less crosslinking density. Epoxy-hardener composition and curing temperature plays key role in developing mechanical properties of final cured coating. The properties of epoxy coating can be modified by blending different types of epoxy, modifying epoxy resin or by using different type of hardeners for crosslinking. Select modifications are listed below.
If a small percentage of the epoxide groups in the epoxy resin is reacted with a long chain alcohol, when cured with the amine, the formed cross-linked films are softer and more impact resistant due to chain termination which reduces crosslink density. Copolymerizing propylene or dipropyl glycol with BPA during synthesis of the epoxy resin provides excellent adhesion and results in lower cross-linked films that are softer. If novalac phenolic resins are reacted with ECH, it produces novalac epoxy resin used as powder coating. Cross-linked novalac epoxy coatings have excellent chemical resistance.2
When epoxy rings are reacted with carbon dioxide, it forms a 5 membered cyclic carbonate ring. With 15% conversion of epoxide groups on the epoxy resin forming cyclic carbonates, it achieves a coating with better impact resistance and hardness when cured with an amine hardener. With the introduction of ethylene oxide oligomer with terminal carbonate rings in to the epoxy resin provides higher amount of bifuctional cyclic carbonates to the modified resin. The impact resistance of resultant coating increases with epoxy-carbonate composition due to formation of hydrogen bonding and the presence of aliphatic ether linkages in resultant resin network. It should be noted that the ester linkages are hydrolysable and reduces the chemical resistance of the coating.
Toughness of epoxy can be improved substantially by modification with reactive liquid rubber, like carboxyl-terminated butadiene-acrylonitrile (CTBN) and amine-terminated butadiene acrylonitrile (ATBN), though the mechanical properties strongly depend on temperature and conditions. If the epoxy resin is modified with silicone rubber, like carboxyl terminated dimethyl siloxanes, the overall toughness of coating improves over a wide range of temperature conditions. A small fraction of added reactive silicone improves the release, slip and coefficient of friction of an epoxy coating. While a high fraction of added high molecular weight reactive silicone gives improved flexibility and strain resistance.
Epoxy esters, synthesized by esterification of the epoxy resin with vegetable oil fatty acids, improves toughness and durability. The formed coating is air dry which means it does not require a curing agent and are used in maintenance and marine application. Use of polyglycidyl ether derivatives of castor oil with BPA improves the flexibility and water resistance of the coating.2
Properties of epoxy coating can also be altered by blending with varying ratios of bisphenol A diglycidyl ether (DGEBA) and polyglycol diepoxide. One can achieve a coating from stiff to flexible without significantly changing the chemical characteristic of coating.
Polysulfides are also blended with epoxy to improve flexibility, but it lowers the physical strength and heat distortion temperature of final coating. If the polysulfide is pre-reacted with the epoxy to form epoxy polysulfide adduct or epoxy terminated polysulfide, it reduces the odor of final coating during application. Such epoxy coatings have improved adhesion towards substrates like aluminum, wet concrete and rusty/oily steel substrates. It also improves the durability of coating in wet environment making these coatings suitable for corrosive environments. Chemical resistance toward oil and fuel is also improved. A cured coating shows improved impact resistance and flexibility at lower temperatures.
Epoxy technologies have been evolving over the last 7 decades, with a variety of new products developed by researchers. The properties and characteristics of epoxy have been enhanced as per requirement over the period of time for diversified applications. With understanding of the science, these coatings are adapting to the new environmental regulation like low VOC.
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